Electrically heatable honeycomb bodies are required, for example, in order to heat components and/or fluids in an exhaust gas purification system in order to achieve or maintain a specific minimum temperature for specific chemical reactions, in particular also for catalytically activated reactions, as a result. This is employed, for example, in exhaust gas catalytic converters, particle filters and/or systems for reducing nitrous oxides.
A typical design of an electrically heatable honeycomb body of this kind is described, for example, in WO 96/10127. EP 1 967 712 also shows the structure of an electrically heatable honeycomb body. Both documents provide the option of supporting (in an electrically insulated manner) an electrically heatable honeycomb body of this kind on an adjacent honeycomb body.
In order to be able to provide a sufficiently high non-reactive resistance for electrical heating, electrically heatable honeycomb bodies of this kind comprise at least one stack of several sheet metal layers (for example foils) which are structured such that they form channels through which a fluid (in particular gas) may flow in an axial direction. Coarsely structured metal sheets, and also finely structured and/or smooth metal sheets are typically layered alternately one on the other for this purpose. The sheet metal layers within one stack are preferably soldered to one another or fastened to one another by diffusion connections at a few (selected) or all contact points.
On account of the stack being looped in an opposite direction in the shape of an S, in the shape of a U or the like, the stack is formed such that it fills a round or oval cross section, wherein adjacent turns of the stack have to be electrically insulated from one another.
U.S. Pat. No. 5,322,673 discloses an electrically heatable honeycomb body in which the stacks of sheet metal layers/foils are arranged in a meandering manner. This arrangement is particularly suitable for honeycomb bodies with non-round or non-oval cross sections, that is to say, for example, with rectangular cross sections.
In this embodiment, constrictions are provided for each change in direction or folding of the stack. In the region of these constrictions, the foils rest smoothly one on the other and, respectively, do not exhibit any structuring. However, producing stacks of foils of this kind is complicated, wherein damage to the foils may nevertheless occur when folding the stacks.